During the production of olefin polymers in a commercial reactor, it is often necessary to transition from one type of catalyst system producing polymers having certain characteristics and properties to another catalyst system capable of producing polymer of different chemical and/or physical attributes. Transitions between similar Ziegler-Natta catalyst systems or other compatible systems is relatively easy. However, where the catalyst systems are incompatible, the transition process is usually complicated. For example, when transitioning between traditional Ziegler-Natta catalyst systems and chromium-based systems, high molecular weight resin agglomerates will form. These agglomerations can form gels in films made with the resulting resin, rendering the final product unacceptable. Consequently, it is desirable to avoid the presence of active Ziegler-Natta catalyst systems when using chromium-based catalysts. Such Ziegler-Natta catalyst systems may comprise a transition metal compound and a cocatalyst, which is often a trialkyl aluminum compound.
In the past, an effective transition between Ziegler-Natta catalyst systems that contain cocatalysts such as trialkyl aluminum compounds and chromium-based catalyst systems was accomplished by first stopping the first catalyzed polymerization process using various techniques known in the art. The reactor was then emptied, recharged and a second catalyst system was introduced into the reactor. Such catalyst transitions, however, are time consuming and costly due to the need to shut down the reactor for an extended period of time.
It would be highly beneficial, therefore, to employ a process for transitioning between incompatible catalysts without the need for halting the polymerization reaction, emptying the reactor and then restarting the reactor with a different catalyst system. It would also be desirable to employ a transition process which reduces the amount of off-grade material produced during the transition, reduces the time for the transaction, increases the robustness and stability of the transition process and avoids the need to open the reactor to charge the seed bed.